The Colorado Plateau has been the site of accumulation and preservation of nonmarine sediments since late Paleozoic time. The climatic conditions have been desert-like for long periods, and wind-blown sand is a common sedimentary type. Much of the alluvial material was carried only relatively short distances and can be related to nearby source areas. The deep and intricate erosion of the region permits excellent three-dimensional views of the sedimentary bodies.
Extensive eolian deposits occur in the Permian, Triassic, and Jurassic Systems. These are mainly interpreted as superposed dune fields. In many instances the edges of the formations are abrupt, and comparison with modern sharply defined dune areas is obvious. Tangential cross-bedding with occasional contorted masses characterize these deposits. Chief interest attaches to the determination of wind directions; apparently the source of most of the,sand lay to the north and northwest.
Fluvial deposits are common above the Pennsylvanian. These offer excellent opportunity to study sedimentary variations resulting from differences in climate, weathering, distance of transport, provenance, and energy relations of stream systems. The common occurrence of uranium deposits in the fluvial sandstones has stimulated geologic investigation. The petroleum possibilities of these beds are also receiving increased attention.
Practically every type of deposit seen in process of formation in modern rivers can be detected in the consolidated rocks. The overbank or flood-plain deposits are of less variety and interest than the channel deposits. All types of bars and channel-fill deposits are present, but those formed during the building of alluvial plains are most common. Apparently, the final composition of a typical fluvial formation depends on the gradient of the streams, the total amount of sediment supplied, and the relative amounts of fine and coarse material.
Internal structures of channel sandstones show great variety and can be related to stream volume and velocity. Ripple mark, festoon cross-bedding, rib and furrow, and lineation are the most common.
Figures & Tables
Geometry of Sandstone Bodies
This volume contains the eight papers presented as a symposium of the Research Committee of The American Association of Petroleum Geologists at the 1960 annual meeting in Atlantic City, New Jersey. One paper presented in the General Session at that meeting, one reprinted paper, and three other solicited papers are also included.
The choice of “Geometry of Sandstone Bodies” as a timely and pertinent subject lor the 1960 symposium was made after an extensive canvass of Research Committee members and about fifty other geologists vitally interested in research in petroleum geology. From a group of about 15 proposed subjects, this one was selected as first choice by almost all those canvassed. Partly because of this high level of interest, the decision was made to attempt publication of the symposium as a special volume of The American Association of Petroleum Geologists. The word geometry in the title probably had several different meanings among the selectors, and for this reason an attempt was made to define the term adequately in order to establish uniformity of communication among symposium participants.
The dictionary definition of the word “geometry” is the science of magnitudes in space. In applying the term to the symposium theme, some modification and interpretation of its formal meaning were needed, and the following definition was therefore proposed for use in this volume—
Geometry of Sandstone Bodies—Spatial relationships of sandstone deposits within the sedimentary framework.
As used in this book, the subject is more than just a three-dimensional study in which thickness is added to areal distribution.